This invention relates to a process for improving the effectiveness of the removal of zirconium from an aqueous zirconium-containing nuclear fuel and/or fissionable material solution in nitric acid in a liquid-liquid process.
A relatively large quantity of zirconium is directly or indirectly formed through the fission of .sup.235 U, .sup.239 Pu, etc. This zirconium is a mixture of stable and radioactive isotopes and belongs to the fission products which represent the greatest obstacle to the reprocessing of spent nuclear fuels and/or fissionable materials by the so-called Purex process for the recovery of unused uranium and plutonium. After dissolution of the spent fuel in nitric acid, the zirconium is present in the fuel solution in a nitrate form which shows chemically complicated and not always reproducible behavior. The extractant (hereinafter referred to as solvent) used in the Purex process is a solution of tributyl phosphate (TBP) in an alkane mixture (kerosene). Acidic butyl phosphates (dibutyl and monobutyl phosphate) and phosphoric acid are formed during the Purex process through radiolytic and hydrolytic decomposition of a small part of the TBP. In the absence of acidic butyl phosphates, the extractability of the zirconium with the solvent is just low enough to enable the zirconium to be separated from uranium and plutonium in the Purex process. Even in low concentrations, however, acidic butyl phosphates increase the extractability of zirconium(IV) salts and it then becomes correspondingly more difficult sufficiently to free uranium and plutonium from .sup.95 Zr after relatively short cooling times of the fuel (less than 1 year). However, after long cooling times when the radioactivity of .sup.95 Zr has decayed (half life 65 d) and decontamination is no longer so important, the difficulties which can arise out of the relatively high chemical concentration of zirconium in the fuel solutions after any cooling period still remain. This is because the zirconium(IV) tends to form deposits (so-called crud) with phosphoric acid and, in some cases, even with acidic butyl phosphates. These deposits seriously affect the fluid dynamics of the extraction steps of the Purex process.
Hitherto, sufficient decontamination of, i.e. removal of Zr from, the solutions to be further processed has only been achieved by expensive repetitions of extraction steps in several clean-up cycles. The formation of crud could not be reliably prevented.